Sung Quek

Image reconstruction of steel reinforcing bars in concrete using Fourier-domain interpolation applied to a sparsely populated data set

High-resolution image generation of bars in concrete using a single coil scanning inductive sensor is a time consuming process. This paper presents a method of generating the image using a sparsely populated data set (SPDS), obtained from a reduced number of scan lines, whose use is justifies by an analysis of the sensor spatial frequency response. Three methods are discussed: linear and cubic spline interpolation in the spatial domain, and Fourier interpolation, all of which are applied to an SPDS. Data for the SPDS are provided from a widely spaced scanning regime implemented in both the x and y-axes. Interpolated values are then synthesized to obtain high-resolution images. The Fourier-based method employs zero padding in the spatial frequency domain and inverse Fourier transformation to obtain higher resolution data in the time/spatial domain. In general, the results obtained by the linear interpolation algorithm are unacceptable since they do not represent the point-spread function of the sensor. The results obtained by the cubic spline and Fourier methods are very satisfactory, with the deviation from the results obtained by the standard high-resolution image generation process being very small. However, the cubic spline method is cumbersome to implement, requiring the computation of a large number of unique polynomials. In contrast, the Fourier algorithm is efficient, straightforward to code and yields an ideal band-limited interpolation. Experiments show that this new methodology is faster than the traditional scanning protocol by at least a factor of 10; a large area scan of 0.5 m2 can thus be produced in 12 min, rather than 2 h. In the future, this technique could be applied to widely spaced solid-state arrays, requiring a fraction of a second for image synthesis.

Multiple layer separation and visualisation of inductively scanned images of reinforcing bars in concrete using a polynomial-based separation algorithm

Inductive scanning systems that exploit eddy current effects for imaging steel reinforcing bar mesh within concrete have been developed and reported by the authors in several publications. Images generated in this manner depict the different horizontal and vertical layers of the mesh within a single, 2D plane. Deeper bars appear as much fainter structures than those closer to the surface for two reasons: first, the signals they generate are weaker, and second, the image is linearly normalised with respect to the much stronger signals returned from the upper bars. This makes depth and dimensional analysis of deeper bars a severe problem. Below we describe a new suite of image processing algorithms that enables the original composite image to be visualised as separate, multiple images representing the various bar layers. This technique is termed polynomial-based layer separation (PBLS). The method also makes it possible to perform brightness-compensation of the lower bars and is a precursor to analysis that allows measurement of the diameters, orientations and depths of the bars. This information is critical for civil structures inspection teams. Knowing the spatial location of the bar peak widths in one layer, curve fitting is applied to calculate the baselines of the bars in other layers of the image. For a two-layer image, the peaks in the lower layer are removed leaving an image of the top bars, and vice versa for the bottom bars. Images of steel bar mesh processed using this PBLS system offer significant enhancements to the qualitative and quantitative properties of the original image data, and in tests described below, is significantly more robust than comparable methods of image segmentation.

A Preliminary Magnetoinductive Sensor System for Real-Time Imaging of Steel Reinforcing Bars Embedded Within Concrete

This paper studies the feasibility of using solid-state magnetoinductive probes for detecting and imaging steel reinforcing bars embedded within prestressed and reinforced concrete. Changes in the inductance of the sensor material are directly proportional to the strength of the measured magnetic field parallel to the sensor. Using a square coil of 300 mm times 300 mm times 2.5 mm, 10-mm rebars can be imaged down up to a depth of 100 mm. Experimental results obtained by scanning steel bar specimens are presented. General performance characteristics and sensor limitations are also investigated.

Inductive and magnetic field inspection systems for rebar visualization and corrosion estimation in reinforced and pre-stressed concrete

Inductive and magnetic field inspection systems are becoming increasing popular for the nondestructive imaging and condition assessment of reinforcing components, such as steel reinforcing bars (rebars) and tendons in reinforced and prestressed concrete structures. In this article, we review the principles of this nascent technology, the research and commercial instruments that are now available, and the directions of future research. Magnetic field imaging technology has in general many potential benefits; it is truly nondestructive and non-invasive, it is non-hazardous, cost-effective and, most important, ignores the concrete matrix in which the reinforcing components are embedded. Most significantly, by analysing the impedance change in an excitation coil, it is also possible to obtain quantitative information (and image data) in relation to regions of corrosion. However, the testing industry has traditionally been reluctant to apply this methodology, for the important reason that the detection range is limited by the rapid attenuation of the magnetic field with increasing distance from the source. This limitation is now being addressed with research into novel coil arrangements, new, more stable and sensitive solid state sensors, and reconstruction algorithms that allow virtual three dimensional reconstruction of embedded components.

Development of a solid-state multi-sensor array camera for real time imaging of magnetic fields

The development of a real-time magnetic field imaging camera based on solid-state sensors is described. The final laboratory comprises a 2D array of 33 x 33 solid state, tri-axial magneto-inductive sensors, and is located within a large current-carrying coil. This may be excited to produce either a steady or time-varying magnetic field. Outputs from several rows of sensors are routed to a sub-master controller and all sub-masters route to a master-controller responsible for data coordination and signal pre-processing. The data are finally streamed to a host computer via a USB interface and the image generated and displayed at a rate of several frames per second. Accurate image generation is predicated on a knowledge of the sensor response, magnetic field perturbations and the nature of the target respecting permeability and conductivity. To this end, the development of the instrumentation has been complemented by extensive numerical modelling of field distribution patterns using boundary element methods. Although it was originally intended for deployment in the nondestructive evaluation (NDE) of reinforced concrete, it was soon realised during the course of the work that the magnetic field imaging system had many potential applications, for example, in medicine, security screening, quality assurance (such as the food industry), other areas of nondestructive evaluation (NDE), designs associated with magnetic fields, teaching and research.

A 1-D Solid-State-Sensor-Based Array System for Magnetic Field Imaging of Steel Reinforcing Bars Embedded Within Reinforced Concrete

This paper describes a linear 1-D solid-state-based magneto-inductive sensor array for detecting and imaging steel-reinforcing bars embedded within prestressed and reinforced concrete. Using a square coil of 300 mm times 300 mm times 2.5 mm and by only measures of the vertical component of the magnetic flux density, pictorial representation of embedded rebars of different diameters and configurations can be imaged down up to a depth of 100 mm. The system is also capable of imaging metallic objects of different shapes. Experimental results obtained by scanning different steel bar specimens are presented.

Modeling Studies on the Development of a System for Real-Time Magnetic-Field Imaging of Steel Reinforcing Bars Embedded Within Reinforced Concrete

This paper addresses fundamental issues associated with the development of a real-time magnetic-field imaging system for nondestructive testing of prestressed and reinforced concrete. Modeling results have shown that with a square coil of 300 times300 times2.5 mm3, 10-mm rebars can be imaged down to a depth of 100 mm. Studies also indicate that the vertical component of the induced magnetic field is most favorable as it can readily be reconstructed to yield geometry and dimensional information pertaining to the rebar structure.

THE APPLICATION OF MAGNETO INDUCTIVE SENSORS FOR NON-DESTRUCTIVE TESTING OF STEEL REINFORCING BARS EMBEDDED WITHIN PRE-STRESSED AND REINFORCED CONCRETE

This paper demonstrates the feasibility of using solid‐state magneto‐inductive probes for detecting and imaging of steel reinforcing bars embedded within pre‐stressed and reinforced concrete. Changes in the inductance of the sensor material are directly proportional to the strength of the measured magnetic field parallel to the sensor. Experimental results obtained by scanning steel bars specimens are presented. General performance characteristics and sensor output limitations are investigated by using different orientations, sensing distance, excitation intensity, bar sizes and geometries.

Damage and corrosion visualization of reinforcing bars embedded in concrete using a new solid-state inductive scanning sensor

The design and use of two new inductive sensors for imaging of reinforcing bars in concrete is described. The first is a Q-sensor, the second a heterodyne sensor. By exploiting the phase and amplitude of the returned signals, it is possible to image the steel bars, detect section loss due to corrosion and image corrosion products directly. A real-time time digital signal processing system has also been developed that performs data pre-processing.

Solid-state multi-sensor array system for real time imaging of magnetic fields and ferrous objects

In this paper the development of a solid‐state sensors based system for real‐time imaging of magnetic fields and ferrous objects is described. The system comprises 1089 magneto inductive solid state sensors arranged in a 2D array matrix of 33×33 files and columns, equally spaced in order to cover an approximate area of 300 by 300 mm. The sensor array is located within a large current‐carrying coil. Data is sampled from the sensors by several DSP controlling units and finally streamed to a host computer via a USB 2.0 interface and the image generated and displayed at a rate of 20 frames per minute. The development of the instrumentation has been complemented by extensive numerical modeling of field distribution patterns using boundary element methods. The system was originally intended for deployment in the non‐destructive evaluation (NDE) of reinforced concrete. Nevertheless, the system is not only capable of producing real‐time, live video images of the metal target embedded within any opaque medium, it ...